1. Field of the Invention
This invention relates to solid rocket motor nozzle ablative composites. More specifically, the present invention relates to the application of addition-polymerization resin systems as the matrix constituent in fiber-reinforced ablative components.
2. Technology Review
The combustion of a solid propellant in a rocket motor creates a hostile environment characterized by extremely high temperature, pressure, mass flows, and turbulent flow. The flame temperature within the motor often exceeds 6,000.degree. F. Pressure within the motor typically exceeds 1,000 psi. Gas velocities typically range from Mach 0.02 in the inlet region to Mach 10+ at the aft end of the rocket motor nozzle. This environment is particularly hostile in a solid rocket motor because its combustion gas typically contains chemical species and particulates which tend to physically and chemically erode exposed rocket motor nozzle components. While the combustion of the rocket propellant is typically brief (i.e., less than sixty seconds), the conditions described above can destroy insufficiently protected or inferior rocket motor nozzles prematurely and jeopardize the mission of the motor.
Parts of a rocket nozzle which are exposed to the high temperatures, pressures, and erosive flow conditions generated by the burning propellant must be protected by an ablative layer of insulation. Various materials have been tried as ablatives, such as silica dioxide, glass, or carbon fiber reinforced phenolics, but reinforced resin composite materials are most commonly used in the severe environment of the nozzle. The reinforced resin composite materials are typically prepared by taking squares or plies of resin impregnated carbon or graphite cloth and positioning them in the desired ply orientation. The nozzle component is manufactured by press-molding, hand lay-up, or tape-wrapping techniques. Tape wrapping using woven cloth prepreg is by far the most common method used for large solid rocket motor (SRM) nozzles. Phenolic resins, such as phenol-formaldehyde resin, are widely used because of their heat resistance, good insulation properties, low cost, and ease of handling and manufacturing.
Despite use for almost 50 years, significant disadvantages remain with phenolic resin systems. For example, phenolic resin systems are a condensation polymerization resin system, that is, they generate volatile by-products, such as water, during cure. These by-products must be removed through vacuum and other means during cure. If the by-products are not adequately removed, low density indications (LDI) in the final ablative nozzle component may result. A LDI in the nozzle component is often caused by resin pockets or pockets of condensation products.
The manufacture of ablative composites for rocket nozzles using phenolic impregnated fiber reinforcement (carbon, graphite, silica, and others) includes extensive inspection of the manufactured components. The evaluation of the components involves very careful visual inspection of the surfaces of every part for any sign of a defect. Very costly (capital and operating) nondestructive evaluation (NDE) methods are also used to characterize the morphology and structure of the entire component. These techniques include X-ray, computed tomography, ultrasonic, electrical resistivity, and other methods developed to find specific types of flaws (i.e., delaminations). These inspections are very labor intensive and significantly increase the cost of manufacturing the ablative composites. If detected during product inspection, a LDI or other defect may lead to rejection and replacement of the nozzle component. Time and materials are wasted, and the overall cost of manufacturing the rocket motor is increased.
If a LDI or other defect is not detected, there is the potential for problems or failure during actual rocket motor firing. Entrapped volatiles can cause pore pressure build-up during motor operation and possibly produce anomalies such as wedge-out, ply lift, sub-char ply separations and pocketing. These anomalies, if severe enough, could significantly affect rocket motor performance.
It will be appreciated that there is a need in the art for fiber-reinforced nozzle ablative compositions which avoid the generation of condensation polymerization by-products, can be easily manufactured without entrapping volatiles within the nozzle ablative composition using conventional manufacturing methods, and do not affect performance detrimentally.
Such fiber-reinforced nozzle ablative compositions are disclosed and claimed herein.